1. Field of the Invention
The present invention generally relates to an electrode for a rechargeable lithium-ion battery and to a method of fabrication thereof.
2. Description of the Prior Art
Lithium-ion batteries traditionally use lithium metal oxides (LiMO.sub.2 where M=Co or Ni) as cathodes and carbon anodes. The batteries are so named because lithium ions are shuttled back and forth on charge and discharge between the anode and the cathode. On charge, the ions are electrochemically extracted or deintercalated from the lithium metal oxide cathode and inserted or intercalated in the carbon anode. On discharge, the lithium ions are deintercalated from the carbon anode and intercalated into the cathode.
Of all the carbon anode materials that can be used, graphite provides the most capacity, intercalating one lithium per six carbon atoms to form the phase LiC.sub.6. More recently, it has been shown that the graphite can be replaced by materials such as SnO or SnO.sub.2 which have almost twice the gravimetric capacity as graphite. The materials are reduced at the anode to form first Sn and lithium oxide. For example, for SnO, the reaction may be written as:
2Li.sup.+ +SnO+2e-.fwdarw.Sn+Li.sub.2 O (1)
This reaction is irreversible, and results in considerable capacity loss on the first cycle due to the formation of Li.sub.2 O. Upon further charging of the cell, the tin alloys with lithium. Several intermetallic lithium-tin phases are formed. Finally, the phase Li.sub.4.4 Sn, the phase of highest lithium content, is formed as follows: EQU 4.4Li.sup.+ +Sn+4.4e.sup.-.fwdarw.Li .sub.4.4 Sn (2)
Since the alloying of tin is reversible, the battery can then cycle reversibly provided that fracturing of the alloyed tin particles does not cause the active material from losing electrical contact with the anode substrate and irreversible capacity loss. This type of capacity loss is a recurrent problem with this type of electrode. The problem may be overcome only by using materials of extremely small particle size that are separated by an electrochemically inert oxide matrix. However, up to the present, techniques for developing such materials of extremely small particle size have not been provided for ductile materials such as tin.
Furthermore, according to the previous state of the art, processes for development of lithium ion batteries or cells begin with SnO as the active anode material in the lithium-ion cell. Charge from the cathode is required to reduce the SnO to Sn. In order to do this, extra cathode material has had to be added to the cell, and the resultant battery has been large, heavy and of unsatisfactory efficiency.